In the manufacture of metal castings it is important to avoid contamination of the metal with non-metallic inclusions. These inclusions are usually oxide phases, and are usually formed by reaction between the metals being melted and the crucible in which they are melted. It has long been an aim of metal casters to avoid such contamination by using crucibles which have minimum reactivity with the melts. However, some alloys, in particular nickel-based superalloys, which may contain substantial amounts of aluminum, titanium, or hafnium, react vigorously with oxide crucibles and form inclusions during melting.
Heretofore there have been two main methods of avoiding contamination from a crucible in metal smelting. One method is "cold-crucible"melting, in which a water cooled copper crucible is used. The metal charge, which may be melted by induction, electric arc, plasma torch, or electron beam energy sources, freezes against the cooled copper crucible wall. Thereafter, the liquid metal is held within a "skull" of solid metal of its own composition, and does contact the crucible wall.
Another method is levitation melting. In levitation melting, a quantity of metal to be melted is electromagnetically suspended in space while it is heated. U.S. Pat. No. 2,686,864 to Wroughton et al. and U.S. Pat. No. 4,578,552 to Mortemer show methods of using induction coils to levitate a quantity of metal and heat it inductively.
Cold crucible melting and levitation melting necessarily consume a great deal of energy. In the case of cold-crucible melting, a substantial amount of energy is required merely to maintain the pool of molten metal within the skull, and much of the heating energy put into the metal must be removed deliberately just to maintain the solid outer portion. With levitation melting, energy is required to keep the metal suspended. In addition, as compared to the surface of a molten bath in a conventional crucible, levitation melting causes the quantity of metal to have a large surface area, which is a source of heat loss by radiation. Additional energy is required to maintain the metal temperature.
For alloys which are mildly reactive with crucibles, such as the nickel-base superalloys referred to above, a process called the "Birlec" process has been used. This process was developed by the Birmingham Electric Company in Great Britain. In the Birlec process, induction is used to melt just enough metal to pour one casting. Instead of pouring metal from the crucible conventionally, however, by tilting it and allowing the melt to flow over its lip, the crucible has an opening in its bottom covered with a "penny or "button" of charge metal. After the charge is melted, heat transfer from the molten charge to the penny melts the penny, allowing the molten metal to fall through the opening into a waiting casting mold below.
By using a small quantity of metal with the proper induction melting frequency and power in the Birlec process, the metal can be "haystacked," or partially levitated, and held away from the crucible sides for much of the melting process, thus minimizing, although not eliminating, contact with the crucible sidewall. Such a process is in use today for the production of single crystal investment castings for the gas turbine industry.
The use of "haystacking" to melt refractory and titanium alloys was tried by the U.S. Army at Watertown Arsenal in the 1950s, using carbon crucibles, but was not successful in eliminating carbon contamination from the crucible, and there was no satisfactory method of controlling the pouring temperature of the metal to the accuracy desired for aerospace work.
These problems have been solved to a large extent by placing the metal charge to be melted within an induction coil, which exerts on the metal an electromagnetic force which is greater near the bottom of the charge than near the top of the charge. The charge is free-standing on a support plate, which has an opening through it through which liquid metal may drain into a mold or other container placed below the support plate as the charge melts. The support is cooled to a preselected temperature. This technique, sometimes referred to as magnetic suspension melting, or MSM, is disclosed in U.S. Pat. Nos. 5,014,769 and 5,033,948, assigned to the same assignee as the present invention. Familiarity with the MSM process disclosed in those two patents to those skilled in the art is assumed, and that process will not be described in detail herein.
It has been found, however, that while the MSM technique works well for many metals, it is not so effective for certain high-melting temperature metals such as steel, titanium and superalloys. It is believed that, because of magnetohydrodynamic flows within the molten metal, the metal develops instabilities which cause the charge of molten metal to collapse prematurely, before the entire charge becomes molten. In such cases, the molten metal runs off the side of the charge, rather than through the opening in the support plate, as it is intended to.
The present invention solves the problem of instabilities in the molten metal and makes the MSM technique effective for the melting of steels and superalloys.